The present invention relates generally to expcsure apparatuses, and more particularly to a catoptric projection optical system, an exposure apparatus, and a device fabricating method using the same. The catoptric projection optical system use ultraviolet (“UV”) and extreme ultraviolet (“EUV”) light to project and expose an object, such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal display (“LCD”).
Along with recent demands for smaller and lower profile electronic devices, finer semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, the design rule for mask patterns has required that an image with a size of a line and space (“L & S”) of less than 0 μm be extensively formed and it is expected to require circuit patterns of less than 80 nm in the near future. The L & S denotes an image projected onto a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus as a typical exposure apparatus for fabricating semiconductor devices includes a projection optical system for projecting and exposing a pattern on a mask or a reticle (these terms are used interchangeably in the present application), onto a wafer. The resolution R of the projection exposure apparatus (i.e. a minimum size for a precise image transfer) can be defined using a light-source wavelength λ and the numerical aperture (“NA”) of the projection optical system as in the following Equation:
  R  =            k      1        ×          λ      NA      
As the shorter the wavelength becomes and the higher the NA increases, the better the resolution becomes. The recent trend has required that the resolution be a smaller value; however it is difficult to meet this requirement using only the increased NA, and the improved resolution expects use of a shortened wavelength. Exposure light sources have currently been in transition from KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm) to F2 excimer laser (with a wavelength of approximately 157 nm). Practical use of the EUV light is being promoted as a light source.
As a shorter wavelength of light limits usable glass materials for transmitting the light, it is advantageous for the projection optical system to use reflection elements, i.e., mirrors instead of using many refraction elements, i.e., lenses. No applicable glass materials have been proposed for the EUV light as exposure light, and a projection optical system could not include any lenses. It has thus been proposed to form a catoptric reduction projection optical system only with mirrors.
A mirror in a catoptric reduction projection optical system forms a multilayer coating to enhance reflected light and increase reflectance, but the smaller number of mirrors is desirable to increase reflectance of the entire optical system. In addition, the projection optical system preferably uses the even number of mirrors to avoid mechanical interference between the mask and the wafer by arranging the mask and the wafer at opposite sides with respect to a pupil. A smaller critical dimension (or resolution) for the EUV exposure apparatus than a conventional one requires a large NA (e.g., NA of 0.2 for a wavelength of 13.5 nm), while it is hard for the conventional 3 to 4 mirrors to decrease the wave aberration. For the increased degree of freedom in correcting the wave aberration, the increased number of mirrors is needed as well as making the mirrors aspheric. As a result, the projection optical system comes to require so many as six mirrors (while the instant application calls such an optical system a six-mirror system). Such six-mirror systems are disclosed, for example, in U.S. Pat. No. 6,033,079 and WO 02/48796.
U.S. Pat. No. 6,033,079 discloses two typical six-mirror catoptric, EUV projection optical systems in its embodiments. These projection optical systems receive light from an object surface, form an intermediate image via four mirrors, i.e., a concave first reflective surface, a concave or convex second reflective surface, a convex third reflective surface, and a concave fourth reflective surface, and re-form the intermediate image on an image surface via a convex fifth reflective surface and a concave sixth reflective surface. Both of these two embodiments arrange an aperture stop on the second reflective surface.
International Patent Publication No. WO 02/48796 discloses three typical six-mirror catoptric, EUV projection optical systems in its embodiments. These projection optical systems receive light from an object surface, form an intermediate image via a concave first reflective surface and a concave second reflective surface, and re-form the intermediate image on an image surface via a convex third reflective surface, a concave fourth reflective surface, a convex fifth reflective surface and a concave sixth reflective surface. Each of these three embodiments provides an aperture stop between the first and second reflective surfaces along the optical axis.
Other prior art that disclose similar optical systems include Japanese Patent Applications, Publication Nos. 2003-15040, 2001-185480, 2002-6221, U.S. Patent Application, Publication No. 2003/0076483, and U.S. Pat. No. 6,172,825.
However, the configurations disclosed in U.S. Pat. No. 6,033,079 are disadvantageous, because the aperture stop is located on the second reflective surface and causes an increased effective diameter of the fourth reflective surface. More specifically, the EUV projection optical system applies a multilayer coating on a mirror surface so as to increase the reflectance, and the reduced incident angle of a ray, i.e., an angle between the ray and a normal of the reflective surface, is suitable for characteristics of the multilayer coating. Since the EUV projection optical system increases an effective diameter of the sixth reflective surface in order to increase the NA and improve the resolution, the fourth reflective surface should be located apart from the optical axis in order to prevent light shielding. Since the embodiments in U.S. Pat. No. 6,033,079 arrange the aperture stop on the second reflective surface, only the third reflective surface introduces the light to the fourth reflective surface apart from the optical axis. In order to reduce the incident angle as discussed, a distance between the third and fourth reflective surfaces should be made large. Because of this large distance, a distance between the object and the fourth reflective surfaces becomes small, so it is hard to make a space for placing a mirror or something. And this increases the light's divergence onto the fourth reflective surface, requires the extremely large maximum effective diameter of 700 mm, and causes a system whose accuracy of finishing is hard to measure.
The configurations described in WO 02/48796 use a concave shape for both the first and second reflective surfaces, and tend to condense the light on a surface close to the object surface. Therefore, the intermediate image is formed near the third reflective surface, and the divergence on the third reflective surface. Then disadvantageously, ripples on a mirror surface, which are formed during a processing operation and air bubbles in the mirror material directly deteriorate imaging performance. In addition, the temperature rise in the reflective surface due to the energy concentration deforms the mirror shape, and dust on the mirror surface is transferred onto the wafer. Moreover, while the second and third reflective surfaces introduce the light into the fourth reflective surface apart from the optical axis from the aperture stop, the light from the second reflective surface to the third reflective surface approaches to the optical axis due to the concave shape of the second reflective surface and is hard to introduce into the fourth reflective surface apart from the optical axis. To solve the problem, the distance between the first and the second reflective surfaces should be made large, so the distance between the object surface and the second surface becomes small. Therefore it is hard to make a space for placing a mirror or something.